1. Field of the Invention
The invention relates to a device for scanning patterns with a light source which illuminates a scanning field on a recording medium, and in particular in which a sensor is formed of a plurality of photoelectric elements for converting the reflected light received from the scanning field, via a lens, into analog electric image signals, in which the light flux emitted by the light source is automatically readjusted for the compensation of interference factors.
2. Description of the Prior Art
Such scanning devices are employed in automatic character recognition systems in order to convert visually readable ciphers or letters of a recording medium, for example, of a document, of a letter or even of a label, directly into a form that can be evaluated by machine and to supply the characters for further processing to a data processing system.
A scanning device of the type generally mentioned above is known, for example, from the German allowed application No. 2,534,235. A photo diode line having a built-in sampling shift register is employed as the sensor surface. An analog-digital converter is connected to the sensor by way of an integrator, the output signals of which represent digitized image signals. These signals are subject to error by differences in the electron yield of the individual photo diodes, by differences in the light-sensitive surfaces of the diodes, by unevenness of the light and other systematic errors. The errors must therefore be eliminated before further processing of the image signals in a recognition device. In the known device, the white background of the read zone of a recording medium to be scanned is directly employed for the simulation of a calibration norm. This uniform surface is scanned for the preparation of correction values for the image signals. A correction value respectively allocated to a specific photo diode is calculated from the scanning signals thus generated and incorrectly deviating from one another by means of an individual comparison with a reference value. In the known arrangement, a gray scale value storage is additionally provided, which intermediately stores the scanned digitized image signals until a first calculation of the correction values is accomplished.
From the foregoing, the following picture is produced. The recording medium to be read must be illuminated in a character recognition device. The brightness respectively returned from the scanning field of the recording medium is imaged via an optic on a sensor surface which converts the brightness into electrical signals. The fluctuation of the returned brightness stems, on the one hand, from the characters on the recording medium and contains the desired information. On the other hand, however, it contains undesired information caused by interference factors which are caused by the aging of the illumination and by the changing diffused reflectance of the background and the like.
In hand-held reading devices which are manually guided over the recording medium, a further interference factor of significance must be taken into consideration, namely the changing distance of the recording medium from the light source. Here, one must assume that the operator is not in a position to continuously maintain nearly identical scanning conditions within a very narrow range of tolerance. On the other hand, in character recognition devices having automatic transport of the recording medium to be scanned, it is possible to eliminate this undesired information more or less by means of alignment techniques, since a rather precisely defined distance of the recording medium from the light source and, in general, only slight reflectance fluctuations of the recording medium can be presupposed for these types of application. The known corrections of the digitized image signals, however, are no longer sufficient in the case of hand-held reading devices.
Here, and in addition to such alignment methods, it is necessary to constantly adapt the intensity of the illumination of the scanning field on the recording medium to the changing scanning conditions. In this connection, an electro-optical control loop for regulating the intensity of the spot of light of a cathode ray tube is known from the German Pat. No. 1,917,006. Heretofore, cathode ray tubes were more often employed as flying-spot scanners because they allow a point of light, sharply focused on the screen of the cathode ray tube, to be easily deflected, at least in one direction. This point of light is imaged by means of a lens onto the scanning field with the recording medium as the picture plane. The reflected light is converted into electrical image signals with very sensitive photoelectric transducers, for example photo multipliers.
The flying-spot scanners find little use today. The cathode ray tube requires a large spatial volume, is only a very weak light source, and therefore requires very sensitive photoelectric elements whose varying sensitivity moreover falsify the scanning result given different light incidence angles of the light return from the scanning field.
In the known electro-optical control loop, the influences of the varying reflectivity of the individual recording medium is compensated, on the one hand and, on the other hand, separately from this, the modulation of the image signals to be traced to the varying angle of incidence of the reflected light is eliminated. A separate control loop is provided for this purpose. A second pair of photo multipliers which are not oriented toward the scanning field, but rather toward the screen of the cathode ray tube, are part of this control loop. The common output of this second pair of transducers is connected to a control circuit in which a signal for controlling the radiation intensity of the cathode ray tube is derived from the common transducer signal by means of comparison with a reference voltage in such a manner that the second pair of transducers emits a signal which is adapted to the reference voltage as much as possible.
Flying-spot scanners which exhibit additional photoelectric transducers in order to generate an adjustment magnitude in a control loop which serves to control the intensity of the point of light on the screen of the cathode ray tube in such a manner that varying angles of light incidence of the reflected light on the photoelectric transducers scanning the field of the recording medium are compensated are therefore to be presumed as known in character recognition devices which utilize automatic document transport.
The principle of the flying-spot scanner with its single deflected light beam requires a completely different scanning process than a scanning device of the type generally mentioned above. Moreover, the control loop known in this connection is also far too expensive in its basic construction for character recognition devices which are to be peripherally employed and be as cost-effective as possible. In the known installation, one has proceeded precisely from that consideration of duplicating the entire arrangement for the photo-electric conversion of the scanning signal in order to create a separate control loop.